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Much AMT-icipated
1mil (0.025mm) Kapton is just about the industry standard for AMTs. It holds true for the modern version of the legendary ESS AMT-1 and AMT-2 and the Eton ER4. Kapton is a premium-performance polyimide film. Improving upon its predecessors Dacron, Mylar and Kaladex, Kapton is the latest triumph of the many polyester fibers which resulted from the modified nylon technology that Dupont has pioneered since the early 1930s. Mylar cones are still used in mini speakers and headphones today and the material at one time (1960s) was the optimal substrate for magnetic audio/video tapes. It was also used in aluminum ribbon drivers as in some older Apogees but soon replaced by the more durable Kapton. Why couldn't it be just Kapton or aluminum? The answer is, two are better than one. You need the durable elasticity and heat/wear resistance of Kapton to squeeze the pleated diaphragm at ultra-high velocities and kinetic energies. At the same time, you need the rigidity and thinness of aluminum to maintain the shape and form of the pleated diaphragm form. More importantly, Kapton is a dielectric. You need the aluminum conductors to respond to the magnets.

Is newer and more durable better? Nelson Pass reminisced: "As I recall, the original Heil diaphragms were polyethylene, not Mylar or Teflon. These later materials were tougher but never sounded as good. The original best-sounding material was made from the packaging material for airline peanuts and may still be in use in the food packaging industry."

Mr. Pass felt that the newer substrate was too tough. Whether or not nostalgic sentiments came into play with that statement is not for me to say. But there indeed is a technical reason to reform the 'newer but sonically inferior' substrate and make it lighter and thinner for faster transient response, lower FMD and expanded bandwidth. According to Daniel Lee, M&D buys the raw 0.0125mm Kapton film directly from their supplier. There are not many manufacturers with the capability of manufacturing 0.5mil polyimide. Hence it is many times more expensive than standard 1.0mil Kapton. Strong and durable as it is, a 0.5mil Kapton is almost impossible to work with given the minute folds required for a Dreams driver. It took countless trial and error sessions before M&D eventually mastered the necessary assembly techniques for building their AMT diaphragms with this super-thin hi-tech substrate. Daniel Lee reiterated just how painstakingly complicated and labor-intensive the process is. But the results are well worth it:

"A 0.0125mm super-thin polyimide (Kapton) as the new diaphragm substrate has been successfully crafted to constitute the DM-4a Dreams AMT driver. Due to the lighter moving mass of the diaphragm, transient response and detail resolution have been improved. This substrate with higher compliance has also significantly extended the low-end response, allowing us to lower the two-way crossover point from the original 900Hz (as in the Ruby, Sapphire and Topaz) to below 700Hz! With a wide 5-octave plus response from 700Hz up to 25KHz ... this significantly helps to lower frequency modulation distortion for the SX woofer.

"The DM-4a is a true 5-octave widebander with low FMD pollution, high output and satisfactory dispersion characteristics over the full operating band. The DM-4a has a large diaphragm (32 x bigger than a typical 1" dome tweeter) and is composed of 32 lightweight micro elements and 14 strong NdFeB magnets. Typical DC impedance of the DM-4a is 4.5 Ohms ±5%. It is completely resistive throughout the entire audio band, without resonance or inductive or capacitive colorations. This perfect impedance characteristic provides a highly stable load to the power amplifier."

AMTs should not be confused with their close cousins of ribbon tweeters and magnetostats, which likewise use ultra-thin aluminum membranes or conductors on a Kapton substrate but not in a three-dimensional pleated fashion. Examples of these flat drivers can be found with Apogee, Magnepan, Eminent Technology, SLS, Genesis and on a much smaller scale (shaped and housed in a metallic module very much like an AMT) in certain hybrid designs by Dali, Verity, Ascendo, Aurum Cantus, JAS Audio, Swan Hi-Vi and more. There are also DIY modules from Visaton, Audaphon/Foundtek, Expolinear, Hi-Vi Research and ATD, the Italian maker of the rare circular dome ribbon tweeter.

With only a few exceptions that can extend downwards to 1.6/1.5kHz or even 850Hz (the Audaphon/Fountek Neo Pro 5i for example has an awesome response of 850Hz to 40kHz), most of these drive units are restricted to well above 2kHz. This might indicate how to reliably manufacture a truly wideband non-pleated ribbon tweeter is beset with very real challenges. How much more complex is an AMT by comparison? Although the legendary AMT-1 was specified as 750Hz to 32kHz (-6dB), not surprisingly a lot of DIYers tend to overload the driver by setting the crossover point at 800Hz whereas the official recommendation was from 1.7kHz on up. Here's another interesting pop-quiz item about non-pleated ribbon tweeters. Due to the gentler excursions and less vigorous pistonic action of the smaller bandwidth-restricted ribbon modules, their membrane thickness can afford to be as waver-thin as 0.009mm (as is indeed the case with the Audaphon JP 3.0). But the data sheet carries a warning: "This ribbon is not as solid as the much thicker one used in the CD 3.0; vacuuming results in lengthening the 0.009 mm thin ribbon and subsequent destruction of the driver" and only recommends it for "small and thin floorstanding or shelf speakers."

Merry Xmax
The SX5.5-0.8 woofer incorporated in the Ruby and Sapphire already set excursion figures to be proud of: "This 5.5" woofer with Mark & Daniel's patented SX technology has an enormous 33mm long magnetic field created by NdFeB magnets. The high efficiency copper voice-coil ribbon in 13mm length is wired onto a 250°C grade Ø36mm TIL glass-fiber former, yielding an amazing ±10mm (0.8") linear excursion." Now imagine what had to be done to push the linear excursion to ±12.5mm! While most people would assume that increasing magnetic flux would also increase pistonic magnitude, M&D thought otherwise. "We encountered many technical challenges when we attempted to further increase linear excursion. It goes well beyond just investing more money on precious magnets. Too strong a total flux created by too large a group of NdFeB magnets will inevitably saturate the magnetic loop and thereby adversely restrict the Xmax (maximum linearity) excursion to our disadvantage."

Without giving away trade secrets, the QMA (Quantified Magnets Alignment) technology in the newly developed 5.5" SX5.5q-1.0 woofer is claimed to "have a 25% excursion improvement over the original SX5.5-0., i.e. an unprecedented ±12.5mm (1") of Xmax... The SX5.5q-1.0 woofer has been assembled for the Maximus-Diamond+ speaker to provide stunning low bass with very high amplitude (the system F3 is an amazing 42Hz), and full Q-control (over the full excursion range."